Two ways to fit your image
Use a preset when you know exactly what size a platform needs — pick one and it crops and scales to that exact size for you. Use custom crop when you want control over the framing: choose any aspect ratio, then pan and zoom the image to get the crop exactly right.
Not sure which dimensions a platform actually wants? Check the image size cheat sheet for current sizes across every major platform.
Splitting a photo into an Instagram grid?
For a multi-tile Instagram grid layout, use the Instagram grid maker instead — it splits one photo into the numbered tiles you need.